1. Field of the Invention
The present invention relates generally to improved fiber optic connectors and, more specifically, to mechanical splice connectors that do not require the use of an index matching gel.
2. Technical Background
Optical fibers are widely used in a variety of applications, including the telecommunications industry in which optical fibers are employed in a number of telephone and data transmission applications. Due, at least in part to the extremely wide bandwidth and the low noise operation provided by optical fibers, the use of optical fibers and the variety of applications in which optical fibers are used are continuing to increase. For example, optical fibers no longer serve as merely a medium for long distance signal transmission, but are being increasingly routed directly to the home, or in some instances, directly to a desk or other work location.
With the ever increasing and varied use of optical fibers, it is apparent that efficient methods of coupling optical fibers, such as to other optical fibers, to a patch panel in a telephone central office or in an office building or to various remote terminals are required. However, in order to efficiently couple the signals transmitted by the respective optical fibers, a fiber optic connector must not significantly attenuate or alter the transmitted optical signals. In addition, the fiber optic connector must be relatively rugged and adapted to be connected and disconnected a number of times in order to accommodate changes in the optical fiber transmission path.
A wide variety of factory and field-installed fiber optic connectors are known in the prior art. It is desired to have an optical fiber connector that is inexpensive to manufacture, easy to install and is capable of withstanding a wide range of environmental factors. In factory-installed connector designs, the connector is coupled with the end of one or more optical fibers during a factory assembly process. Factory installation of the fiber optic connectors onto the end of the optical fibers allows for increased accuracy in the assembly and construction of the connector and avoids the environmental and technical problems associated with field installation.
However, it is not always possible to factory install fiber optic connectors on the termination ends of optical fibers in every situation. For example, in widely-deployed networks, the optical fiber that terminates at the customer's premises, known as a field fiber, can vary in the desired length. Similarly, optical fiber installed within a structure may require optical fiber runs ranging from just a few feet to several hundred feet. Furthermore, the physical space limitations may not permit storage of excess fiber length that naturally results when installation is limited by a small number of available fiber lengths. With such varying lengths and the desire to minimize any excess slack on the ends of the optical fiber runs, it is simply not practical to install factory connectors on the fiber because of the uncertainty and variability in the length of field fiber.
Consequently, field-installable optical fiber connectors have been developed which can be coupled onto an end portion of an optical fiber in the field once the particular application and length of the optical fiber has been determined. Although alternative types of connectors are available, one of the most common forms of field-installable connectors is the mechanical splice connector. Mechanical splice connectors create a physical mating between the ends of mating optical fibers. Frequently, these mechanical splice connectors use an internal fiber contained within the connector to mate to the inserted field fiber within the connector. The internal fiber, commonly known as a “stub fiber” or “fiber stub”, usually extends from about the end of a ferrule to approximately halfway along the length of the connector. This stub fiber is factory polished at the ferrule end, enabling the ferrule and stub to be readily mated with another connector after installation of the connector. The other end of the stub fiber may be either cleaved or polished in the factory and provides a mating surface for engaging with an inserted field fiber.
Performance of an optical junction between two fibers includes several important parameters, such as forward power loss (usually referred to as insertion loss) and reflected power (reflectance or return loss). Insertion loss decreases the power available at the receiver, increasing the likelihood of data disruption or corruption. Reflectance causes noise in the optical signal and can affect transmitter function. Insertion loss is primarily affected by lateral misalignment of fibers at a junction. Angular misalignment and separation between fibers also contribute to insertion loss. Reflectance is primarily affected by a change in index of refraction along the optical path, such as would happen if the light signal passes from glass (n=1.468) to air (n=1), and can be calculated using Fresnel's equations. A junction with large reflected power will also suffer measurable insertion loss as power is reflected instead of transmitted. For example, a fiber to air interface with a reflectance of approximately −14.7 dB, will incur approximately a 0.3 dB insertion loss due to reflection. For reference, a good optical junction such as a precision splice or connector will incur a 0.05 to 0.25 dB insertion loss, and −40 to −65 dB reflectance.
One of the more important aspects of installing a mechanical splice connector is ensuring that the stub fiber and inserted field fiber are accurately aligned to ensure minimum insertion loss across the fiber-fiber interface. A number of mechanisms are known in the prior art to accomplish the task of accurately aligning the optical fibers, including V-grooves and camming mechanisms. Alignment mechanisms in the art ensure that the core of the fiber stub and the core of the field fiber are accurately aligned and the field fiber is then locked into position. After the optical fibers are aligned and the field fiber is locked into position, the alignment between the fiber stub and the inserted field fiber must be precisely maintained to provide a consistent, reliable connection. Proper alignment however ensures only good insertion loss. To minimize reflectance, index of refraction changes must be eliminated from the optical path.
In order to accomplish this in prior art mechanical splice connectors, it is known to fill the connectors with index matching gel. Index matching gel has an index of refraction that is very close to that of the core of the optical fibers when the temperature of the connector is maintained at room temperature. If the fiber stub and the field fiber are not precisely contacting due to minute variations in cleave angle or surface topography of the fiber ends, the index matching gel enhances the transfer of the optical signal between the fiber stub and field fiber by eliminating air gaps which would yield reflections and insertion loss. The index matching gel therefore results in a smaller insertion loss of the optical signal within the connector. The resulting connector provides a reliable and consistent optical connector when the temperature of the connector is maintained within a small range. Another method of creating and maintaining a physical connection without index of refraction change between the fiber stub and the inserted field fiber is to use an axial load on either the field fiber or fiber stub forcing the respective fiber in engagement with the other to eliminate any air from the interface even under varying temperature conditions. This is commonly referred to as creating “physical contact” between the optical fibers. One method known in the art of providing the axial load is to use spring force within the optical connector. However, both of these methods have disadvantages.
A disadvantage of using index matching gel is that the refractive index of the gel varies with the temperature of the gel and with the wavelength of the transmitted light. Although the refractive index of any material may vary with changes in temperature, liquids and gels, such as the index matching gel, are more susceptible to changes in refractive index than a solid, such as an optical fiber, for a given change in temperature. The net result is that as the temperature of the connector diverges from room temperature the respective refractive indices of the optical fibers and the index matching gel diverge as well. Even small differences in the refractive index of the index matching gel and the optical fiber can result in significant increases in reflectance at the interface. Therefore, while index matching gel is extremely effective in indoor applications where the temperature of the connector does not vary significantly, it is a poor choice for outdoor applications where the temperature variations can cause the internal reflectance of the connector to be poor and unreliable. The wavelength dependence of index of refraction in index matching gel makes it more difficult to precisely match the index of refraction of the gel to the fiber in order to achieve low reflectance at multiple wavelengths. This leads to a compromise value of gel index of refraction even at room temperature. Thus, in theory, better performance can be achieved with physical contact than with index matching gel, even at room temperature.
In prior art designs that eliminate the use of index matching gel, the spring loading of a field fiber or installed field fiber to provide an active force between the two also has its disadvantages. Most notably, the use of a spring load requires the inclusion of a spring within the design of the optical connector. As the optical connectors are reduced in size to increase the number of connectors that may be fitted within a given space, the difficultly in designing an optical connector with a spring load increases. Additionally, as the number of components utilized within an optical connector increases, the corresponding cost per unit also increases. It would be desirable to provide an optical connector that can provide an axial load on either the field fiber or fiber stub without the use of a spring. The resulting optical connector would have the same properties as a spring-loaded optical connector, but with less complexity and lower cost.
Accordingly, it would be advantageous to have a mechanical splice connector that is robust and has predictable reflectance properties across a wide range of temperatures. Such a connector should eliminate the use of index matching gel to enhance temperature stability, but should not rely on the complexity of a spring-loading mechanism to maintain physical contact between the field fiber and the fiber stub. The connectors known in the prior art do not address these needs.
In view of the aforementioned shortcomings, improved apparatus and method for performing mechanical splice terminations are needed.